With the rapid advance of the electrical, electronics, and computer industries, there is the gradual increase in demand for a secondary battery with high performance and high safety. Especially, an electric automobile is actively researched and studied in response to the increased interest for the environments. The secondary battery is one of the most important elements of the electric automobile.
The secondary battery includes an anode, a battery separator, and a cathode. For improving efficiency of the secondary battery, it is necessary to obtain the thin battery separator and to secure high porosity and good thermal stability for the battery separator.
For the good thermal stability, the battery separator has to perform a shutdown function, and have a high short-circuit temperature. At this time, the shutdown function is to prevent an additional heat generation through a current cutoff by closing up minute pores of the battery separator if a temperature inside the secondary battery is higher than a predetermined value. The short-circuit temperature means a temperature at which the battery is short-circuited by the battery separator being shrunk, decomposed, and broken. If the battery separator has good heat resistance, the short-circuit temperature is raised.
A typical example of the related art battery separator is a battery separator prepared by polyolefin such as polyethylene or polypropylene. The polyolefin battery separator having a low melting temperature is advantageous in that the shutdown function is properly performed even in case of the battery whose temperature is abnormally high. However, since the short-circuit temperature is too low, it is difficult to apply the polyolefin battery separator to the secondary battery for the automobile.
In order to prevent the battery from being short-circuited at a high temperature, there has been proposed a battery separator prepared by polyethylene terephthalate. The battery separator of polyethylene terephthalate has a short-circuit temperature which is higher than the short-circuit temperature of the polyolefin battery separator, to thereby lower a possibility of short-circuit in the battery by the broken battery separator. Meanwhile, on the assumption that the battery separator of polyethylene terephthalate having too high melting point is applied to the battery, if the battery is exposed to the abnormal surroundings of high temperature, the shutdown function is not performed. Typically, since the battery separator of polyethylene terephthalate is manufactured by a non-woven manufacturing process, it is difficult to obtain required thinness and porosity level.